A two component polyurethane coating consisting of a hardener component of a polyisocyanate compound and a resinous component of a hydroxyl group containing film-forming resin is being watched with keen interest in various technical fields including the auto-repair, wood processing and other industries, because such coatings do not require a high temperature baking operation.
In such a coating, it is highly desired to be of a fast-drying nature from the viewpoint of reduction in processing steps and energy consumption.
Various attempts have been, therefore, made to give the desired fast-drying characteristics to two component polyurethane coatings, but no satisfactory solutions have been yet found. For example, when one attempts to utilize, as a resinous component, an acrylic resin with a higher molecular weight or a higher glass transition temperature (Tg), there occurs such problems that, since a non-volatile matter in the spray is in a lower order, the coating must be applied many times and, moreover, the coating thus obtained is, in general, poor in appearance.
In attempts to compound each resin with a cellulosic resin, such as nitrocellulose, cellulose acetate butyrate (CAB) and the like, there are similar problems as above-mentioned. When an increased amount of catalyst is used, the pot-life of the resulting composition is proportionally decreased and the coating thus obtained is poor in appearance. Further, when an attempt is made to use, as a hardener an aromatic polyisocyanate compound, there is the problem of poor weather resistance. Thus, none of the aforesaid attempts have succeeded in producing fruitful results.
Recently, the so-called gellated resin particles having three-dimensionally crosslinked structures and being substantially insoluble in an organic solvent have been proposed. If the gellated resin particles are in the two component polyurethane coating system, it would be probable that the desired fast-drying might be obtained therewith from the combination of a pigment filler effect as plastic pigment and hard resin effect of the crosslinked polymer particles.
However, the heretofore proposed crosslinked polymer particles were merely developed for the purpose of high solidification or rheology control of a solvent type coating composition, thereby overcoming the drawbacks of sagging and cratering tendencies and of deficient weather resistance or the like. Even the slightest consideration had been made as to the possible use of these crosslinked polymer particles in the room temperature curing type polyurethane coatings. Therefore, even when examined with a two component polyurethane coating to be cured at room temperature, it was not possible to produce a product which satisfies with every aspect of application characteristics, drying properties and dispersion stability. The coating thus obtained was in fact poor in gloss, transparency and finishing appearance. Under the circumstances, it is a principal object of the invention to provide a two component polyurethane coating containing crosslinked polymer particles, which is excellent in dispersion stability, application characteristics and drying fastness, capable of resulting in a coating with excellent gloss, transparency and appearance, as well as excellent polishing resistance, and which is particularly useful as an auto-repair paint and the like.